A number take exam; naturally it's hard (5)
I believe the answer is:
coral
'naturally it's hard' is the definition.
I can't tell whether this definition defines the answer.
'a number take exam' is the wordplay.
'a number' becomes 'c' (C is an example).
'take' means one lot of letters go next to another.
'exam' becomes 'oral' (spoken academic test).
'c'+'oral'='CORAL'
Can you help me to learn more?
(Other definitions for coral that I've seen before include "Reef material" , "Creature building reef" , "Calcareous skeletal remains forming reefs" , "Pinkish-orange colour" , "Marine substance, often forming reefs" .)